1. Field of the Invention
The present invention relates to a bandgap reference circuit, and particularly to a bandgap reference circuit for generating a low reference voltage.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a perspective diagram of a prior art bandgap reference circuit 10. The bandgap reference circuit 10 is used for generating a reference voltage VREF1. The bandgap reference circuit 10 comprises a cascode current mirror 12, and a start circuit 14 composed of transistors M9, M10 and M11 for starting up the bandgap reference circuit 10.
When the current passing through the bipolar junction transistors (BJT) Q1 and !2 is equal to each other, by means of the transistors M5 and M6, the voltages of the nodes N5 and N6 are equal to each other. When the size of the BJT Q2 is larger than that of the BJT Q1, and the voltages of the node N6 and N5 are equal because of the cascode current mirror, a first current directly proportional to the surrounding temperature will be outputted from the transistor M7 and M8. Because the emitter-base voltage of the BJT Q3 is inversely proportional to the surrounding temperature, a reference voltage VREF1 irrelevant to the surrounding temperature will be generated when the first current passes through the resistance R2 and bipolar junction transistor Q3.
The cascode current mirror 12 makes the lowest power source path of the bandgap reference circuit 10 be M2-M4-M6-R1-Q2, and the voltage value of the reference voltage VREF1 is (2Vtp+3Vds,sat+VR1+Veb). Vtp is a threshold voltage of a P-type transistor and is about 0.7V. Vds,sat is the lowest voltage of a P-type or N-type transistor when operating in the saturation region and is about 0.3V. VR1 is a cross-voltage of the resistance R1 and is about 0.1V. Veb is the emitter-base voltage of the transistor Q2 and is about 0.6V. therefore, the lowest voltage value of the reference voltage VREF1 is about 3.0V.
Please refer to FIG. 2. FIG. 2 is a perspective diagram of another prior art bandgap reference circuit 20. The bandgap reference circuit 20 is used for generating a reference voltage VREF2. The bandgap reference circuit 20 comprises an operational amplifier 22, resistances R1R2R3, and bipolar junction transistors Q1 and Q2 so as to form a circuit having a temperature compensation function the same as that of the bandgap reference circuit 10. The operational amplifier 22 is only suitable to the input of the P-type transistor because the operational amplifier input common mode voltage is limited by the bipolar junction voltage which is about 0.6V. So, the voltage value of the reference voltage VREF2 is (Vtp+2Vds,sat+Veb) of which the lowest voltage is about 1.9V.
Nowadays, most of the portable electric devices use mixed integrated circuits, such as analog-to-digital converters (ADC), digit-to-analog converters (DAC) and so on, which have to use relatively low reference voltages, such as 1.5V. Because the reference voltages VREF1, VREF2 generated by the bandgap reference circuits 10, 20 are about 3.0V, 1.9V, respectively, which are relatively high, there are problems produced when applying the reference voltages VREF1, VREF2.